This invention relates to an intrusion detection system and in particular to dual-technology detectors wherein the signals generated from a Passive Infra Red (PIR) sensor and a microwave sensor are processed to detect a human intruder and reject pets and other noise producing sources.
The use of dual-technology detectors in security systems is well known in the prior art. The dual-technology detectors produce a number of positive results including detection of trouble conditions, accurate adjustment of thresholds, and rejection of false alarms. Security systems of today typically use a combination of a PIR sensor, a microwave sensor, and/or an ultrasonic sensor to achieve these results. The detection of trouble conditions is achieved by using one type of sensor to detect an intruder and a second type of sensor as a redundant detector that allows the system to monitor the first sensor for accuracy in detecting the intruder. U.S. Pat. No. 5,504,473 discloses an intrusion detector of this type. Accurate adjustments of thresholds are achieved by using a second sensor with a low threshold value to adjust the threshold value of a first sensor. If the second sensor is constantly above its threshold, the threshold of the first sensor is increased. This type of adaptive threshold system allows the threshold of the first sensor to be as low as possible without producing false alarms. An event detection system with an adjustable threshold generator is disclosed by U.S. Pat. No. 5,471,194.
The most prevalent reason for using dual technology detectors is the rejection of false alarms, which may be generated by environmental factors, electrical noise, or pets. Security systems that use dual technology sensors to reject false alarms require both sensors to detect an event before an alarm is generated. The dual sensors reduce false alarms due to environmental factors because both sensors will detect an intruder, but only one sensor will react to an event produced by the environment. For instance, a PIR sensor, which detects changes in temperature, will detect the heating system being turned on, but a microwave sensor will not detect the change in temperature, and therefore there will be no alarm condition. The dual sensors also reduce false alarms due to spurious noise because the noise is typically random and is less likely to occur in both sensors at the same time. Lastly, reduction of false alarms due to pets is dependent on the field of view of the two sensors and the setting of the threshold levels of the comparators. An animal presence is different from a human intruder in that the animal is lower to the ground and the temperature of the animal is lower due to its skin being covered with hair. The dual sensors of the prior art discriminate between an animal and a human by having one sensor's field of view above the floor by a few feet and setting the threshold level of the comparator to a value that is exceed by a signal generated from a human intruder, but not exceeded by a signal generated from an animal presence. In this system the sensor with the higher field of view would not detect the pet and therefore not cause an alarm condition. U.S. Pat. No. 5,670,943 discloses an intruder detector of this type. Unfortunately, this method may false alarm on pets that are very large or that have very little hair and may not alarm on intruders that are low to the ground.
The rejection of false alarms is accomplished by looking for a signal to be produced from both detectors at the same time or in a sequence of time. In some prior art systems, such as U.S. Pat. No. 5,107,249, the output of each detector is fed to separate comparator circuits where the detector outputs are compared to threshold values and the comparator outputs are a high level when the detector outputs are above the thresholds. The comparator outputs are logically ANDed to produce an alarm. If either detector signal is not above its threshold, or the signals do not occur simultaneously, the alarm is not sounded. In other prior art systems, such as U.S. Pat. No. 5,581,236, the time between each comparator output signal is checked to occur within a certain limit. These systems are an improvement over the other prior art systems because they allow for difference due to the sensors' detection rates and differences in the sensors' field of view.
The problem with these systems is that they do not account for differences in the signal strength due to the trajectory of the intruder. Certain trajectories cause one detector to have a high output signal while the other detector will have a minimal output signal. For instance, if an intruder is walking directly towards a PIR sensor, the sensor will be less likely to detect a change in temperature than if the intruder walked in, across, and out of the zone the sensor was covering. In this system the PIR would not detect the intruder, while the microwave sensor would have a strong signal that would not cause an alarm condition.
It is therefore an object of the present invention to provide an intrusion detection system that uses dual sensors to detect human intrusions and reject false alarms due to pets and other noise producing sources.
It is a further object of the present invention to provide an intrusion detection system that will alarm when a high signal is present in one sensor and a low signal is present in the other sensor.
It is still a further object of the present invention to provide an intrusion detection system that distinguishes between a human intruder and a pet presence by processing the signals from both detectors using a microprocessor.
It is still a further object of the present invention to provide an intrusion detection system that may be customized to take into account the trajectory patterns at an installation site.
It is a further object of the present invention to provide an intrusion detection system that may be customized to reject a customer's pet.